Page 71 - Physical chemistry understanding our chemical world

38 INTRODUCING INTERACTIONS AND BONDS

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solid CO 2 at this temperature is about 30 cm . We deduce that CO 2 does not obey
the ideal-gas equation (Equation (1.13)) below its freezing temperature, for the very
obvious reason that it is no longer a gas.
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SAQ 2.1 Show that the volume of 1 mol of CO 2 would be 15 200 cm at
p O and −90 C (183 K). [Hint: use the ideal-gas equation. To express this
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answer in cubic metres, you will need to remember that 1 m = 10 dm 3
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and 10 cm .]
Although solidifying CO 2 is an extreme example, it does show how deviations
from the ideal-gas equation occur.

How is ammonia liquefied?

Intermolecular forces

Compressing ammonia gas under high pressure forces the molecules into close
proximity. In a normal gas, the separation between each molecule is generally
large – approximately 1000 molecular diameters is a good
We sometimes call generalization. By contrast, the separation between the molecules in
a solid or a liquid a a condensed phase (solid or liquid) is more likely to be one to two
‘condensed phase’. molecular diameters, thereby explaining why the molar volume of
a solid or liquid is so much smaller than the molar volume of a gas.
As a direct consequence of the large intermolecular separations,
‘Intermolecular’ means we can safely say no interactions form between the molecules in
‘between molecules’. ammonia gas. The molecules are simply too far apart. We saw
in the previous chapter how the property known as pressure is
a macroscopic manifestation of the microscopic collisions occurring between gas
particles and, say, a solid object such as a container’s walls. But the gas particles can
also strike each other on the same microscopic scale: we say the resultant interactions
between molecules are intermolecular.
Intermolecular interactions only operate over relatively short dis-
A ‘formal bond’ involves tances, so we assume that, under normal conditions, each molecule
the permanent involve- in a gas is wholly unaffected by all the others. By contrast, when
ment of electrons in the gas is compressed and the particles come to within two or three
covalent or ionic bonds; molecular diameters of each other, they start to ‘notice’ each other.
see p. 64. Interactions We say the outer-shell electrons on an atom are perturbed by the
between molecules charges of the electrons on adjacent atoms, causing an interaction.
in a compressed gas We call these interactions bonds, even though they may be too
are temporary.
weak to be formal bonds such as those permanently connecting
the atoms or ions in a molecule.
The intermolecular interactions between molecules of gas are generally attractive;
so, by way of response, we find that, once atoms are close enough to interact, they
prefer to remain close – indeed, once a tentative interaction forms, the atoms or